Marula

Sclerocarya birrea

Origin:

Northern South Africa and eastern Botswana. It is widely distributed in Africa, from Ethiopia and the Sudan in the north to Natal in the south, and from South West Africa to Mozambique in the east, mainly in savannah woodland.

Climate:

It prefers a warm, frost-free climate and is sensitive to frost. If planted in areas where there is mild or occasional frost, it must be protected for at least the first few growing seasons. It does well with an annual rainfall of between 650-800mm pa, concentrated in the summer rainy season. Marula is moderately drought resistant.

Plant Description:

It is a moderately large (15 to 18m in height), deciduous (leaves turn yellowy-green to pale yellow before abscission), single-stemmed tree with a grey mottled bark. The crown is dense, round and wide-spreading. The normally straight, round stem generally subdivides about 3 or 4m above ground level, and often becomes as much as 60cm in diameter. The young twig terminals are abnormally thick and digitaliform, slightly rough because of small scale stipules which persist for some time, and are brown to browny-green in colour. The twigs display noticeable leaf scars and retain their grey to pale grey colour even on thick stems. The bark on old stems is fairly smooth and peels off sporadically in rather large, flat, irregular, more or less round discs, thus exposing the underlying yellowy bark which gradually turns grey. The stems therefore appear to be pale grey with yellow patches. Just under the cork-like bark layer the phloem is red and the xylem pale red. The leaves are borne close together at the ends of the twigs. They are arranged spirally and may become as much as 30cm in length but are usually about 15cm. Leaves are imparipinnate and generally consist of three pairs of opposite, lateral leaflets and a terminal one. The leaflets are subelliptic to lanceolate, acuminate at the apices, usually slightly asymmetrical and cuneate at the bases but occasionally broadly cuneate. They are about 6cm in length and 3cm. in breadth. They are moderately hard and fairly brittle, pale green to green and shiny above but dull, pale green beneath, marginally entire usually somewhat undulate, and sometimes curved upwards or downwards lengthwise. They are also smooth, glabrous, and usually pendent. The veins are quite noticeable on both surfaces with the midrib raised on both sides of the blade and the secondary veins only slightly exserted above. The petioles are pale green to yellowy green in colour, about 1.5cm long, flattened along their upper sides, glabrous and markedly broadened at their bases.

Relatives:

A member of the Anacardiaceae family. Other members of this family are mango, cashew apple, pistachio, ambarella. There are only two species in the genus and three subspecies of marula – subsp birrea, caffra and multifoliolata separated on inflorescence and leaflet characters.

Soils:

Not particular about well-drained soils but has a preference for sandy soils, and is salt tolerant.

Propagation:

Seeds, and truncheons 100-150mm in diameter and 2m long can be planted in the spring. Seed is possible but the juvenility period can be 8-10 years and only half will be female. Soaking the seed for 1-2 days before sowing can improve germination. Alternative vegetative techniques of cuttings and grafting have also been used to ensure gender and fruiting characteristics, but these techniques have also not been without problems. Loosening the operculum significantly improves germination.

Cultivars:

Although improved clones producing large fruits up to 100g in weight and with a variety of skin colours have been selected in South Africa, there are no named cultivars.

Flowering and Pollination:

With infrequent exceptions, marula is dioecious. Inflorescences are axillary, with males being racemes of small groups of small reddish flowers and females reduced to a spike of 1-4 reddish flowers. They are all pentamerous with sepals and petals about 5mm long. Each male flower has 15-25 stamens surrounding a fleshy disc; female flowers have a sub-globular ovary with 2-3 single ovule locules surrounded by 15-25 staminodes. Flowering takes place over a few months during dry periods in spring and individual flowers open in the morning. There is little fragrance but sufficient nectar to attract bees, the principal pollinating agents. Anthers dehisce over 24-36 hours and pollen is viable for up to 48 hours. The stigmatic surfaces in open female flowers are receptive for up to 72 hours and secretion of nectar ceases with fertilization.

Cultivation:

Its usual occurrence is in the wild and around villages and roads. There have been a few efforts at making plantations, but no cultivation information is readily available. In areas with dry summers, the trees do better if some water is supplied.

Wind Tolerance:

Likely to be good.

Pruning:

A fast-growing tree, with a growth rate of up to 1.5 m/year. Prune branches that have borne fruit after harvesting to encourage new growth. Coppicing is a suitable practice.

The Fruit:

The fruits are roughly plum-sized, but there is considerable variation of fruit size, flavour, flesh and seeds from tree to tree. Fruit is borne in clusters of up to 3 at the end of the twigs and always on the new growth. Fruits are a round or oval drupe, usually wider than it is long, with a diameter of 30-40mm. The shape and number of seeds per stone determine the final shape of the fruit. Marula fruit has a thick, tough and leathery exocarp with tiny, round or oval spots, enclosing a juicy, fibrous and fleshy mesocarp that adheres tightly to the stone and can be removed only by sucking. The flesh tastes tart, sweet and refreshing, although the fruit has a slight turpentine-like aroma and can give off a very unpleasant smell when decaying. Each fruit contains an exceedingly hard stone, which is covered by fibrous matter. It is usually trilocular, but locules plus seeds range in number from 0 to 4. Each seed locule contains a single flattened, delicate seed, 15-20mm long, filling the entire cavity, which is sealed by a round, hard disk, the operculum, that protects the embryo until germination.

Fruit Production and Harvesting:

Seedlings start to bear in 7-10 years and reasonable crops are produced in 15-20 years with the yield continuing to increase. Fruiting takes place in the summer. The fruits abscise when ripening commences so that final ripening takes place on the ground and the fruit turns yellow. In the wild, fruits often are dispersed by flowing water. Fruits can be stored for two weeks at 20°C and longer at 12°C; temperatures below 4°C cause chilling injury. Fruits bruise easily and kernels have a short life once separated.

Fruit Uses:

All parts of the fruit are edible. The pulp can be consumed raw or boiled into a thick, black consistency and used for sweetening porridge. Vitamin C content is slightly more than that of oranges. The pulp also makes an excellent conserve and a delicious amber-coloured jelly. The kernels, while only about 2% of fresh weight, are high in protein (28-31%), oils (56-61%) and magnesium. They are regarded as a delicacy in countries such as Tanzania and Zambia where they’re commonly used to supplement the diet during winter or drought periods. They are mixed with vegetables or meat or may be pounded and made into a cake before consumption. However, extracting kernels from the hard stone is difficult.

Pests and Diseases:

Psyllid mites are the major pest, as well as aphids, white flies and thrips. In the field, borers and termites can cause problems. Small seedlings are vulnerable to fungus.

Comments:

They make excellent shade trees for (large) gardens and parks. Truncheons strike readily and can be used to make living fences. If you manage to source marula they could well be seedlings, so you’ll need a number of plants to ensure you have at least one male and a female, and for confirmation you’ll have to wait several years. As they’re a sub-tropical species the south west WA climate should not be a handicap. They are a tough, deep-rooted and drought tolerant plant. There is a small plantation of marulas at the Drylands Permaculture Farm near Geraldton.

Instructions for preparing the marula stone for planting: clean the dry seed, scouring the dried, fibrous flesh from the hard shell with a small electric wire wheel. The locule 'caps' then become visible. These are close-fitting covers over each embryo. Carefully loosen each cap with a small screwdriver that has been ground down a little, but don't remove it totally. Plant in good, loose potting mixture.